1. Field of the Invention
This invention relates to apparatus and methods for use in determining the optical transmission factor or density of translucent elements, such as, for example, optical filters.
2. The Prior Art
It is known to determine the optical transmission factor, or density, of translucent elements, such as filters for use in testing photographic materials during manufacture, by using the inverse square law bench. As is well known, the inverse square law states that the intensity of illumination due to a point source is proportional to the square of the distance from the source. With the inverse square law bench, the intensity of a source is measured at distance R.sub.1 with a detector. The translucent element whose transmission factor is to be determined is then inserted between the source and the detector. The detector is then moved towards the source until it senses an intensity equal to that which it sensed at distance R.sub.1 without the element in the path. The distance at this new location of the detector is R.sub.2. The transmission factor of the element is given by: EQU R.sub.2.sup.2 /R.sub.1.sup.2
If the inverse square law bench is to be of reasonable size it has a very limited dynamic range. In order to extend the dynamic range, it is known to insert a standard filter, or filters, in the path between the source and the detector during the portion of the determination in which the test element is not in the path. This procedure is sometimes termed "bootstrapping". The use of standard filters in this way has been found to introduce errors and has made the test procedure costly on labor.
It is an object of the present invention to reduce the problems of the prior art by providing method and apparatus for determining the transmission factor or density of translucent optical elements which are both more accurate and less labor intensive.